How can a Diffuse material not be effected by light?
This is simply done by setting the color to black. The light of a diffuse material with a roughness of 0 is calculated using the Lambert lighting model.
This model calculates the light that an object reflects as follows:



where  is the intensity of the diffusely reflected light,  is the material's color and  is the intensity of the incoming light,  is the surface's normal vector and  is the normalized vector from the surface point to the light source.

For typical colors  this will give you light areas where the surface faces the light source and darker areas where the surface points away from the light source. But since in your material the color is a pure Black (RGB (0,0,0)) the red green an blue light component will always be multiplied by 0. Thus the resulting surface color will always be black as well, no matter how strong the lightsource is.